Self-adjusting electrochemical sensor

ABSTRACT

A gas detector with a compensated electrochemical sensor exhibits altered sensitivity in response to decreasing stochastic noise in an output thereof. A gain parameter can be adjusted to alter sensitivity. A life-time estimate can be made based on sensitivity.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the benefit of the filingdate of Ser. No. 11/868,318 filed Oct. 5, 2007 and entitled,“Self-Adjusting Electrochemical Sensor”, now U.S. Pat. No. 7,846,320issued Dec. 7, 2010, which is a divisional of U.S. patent applicationSer. No. 10/925,750 filed Aug. 25, 2004 and entitled “Self-AdjustingElectrochemical Sensor”, now U.S. Pat. No. 7,297,242 issued Nov. 20,2007.

FIELD OF THE INVENTION

The invention pertains to gas detectors. More particularly, theinvention pertains to gas detectors having age-compensatedelectro-chemical sensors.

BACKGROUND OF THE INVENTION

Depending on the circumstances it can be desirable and/or particularlyimportant to be able to sense the presence of various gases which mightbe dangerous or explosive. These include carbon monoxide, carbondioxide, propane, methane, as well as other potentially explosive gases.

A variety of sensors are known which can detect various gases. Thesesensors are based on different technologies and have differentperformance characteristics and different cost characteristics. Onetechnology of ongoing interest is represented by electrochemicalsensors. This class of sensors is potentially reliable and inexpensive.

Electrochemical sensors can be designed so as to be responsive to a gasof interest and to be highly sensitive. They respond to a gas ofinterest with a respective output current. However, such sensors have azero output current failure mode and zero output current in the absenceof the selected gas. Because there is no specific failure indicator,external circuits have to be designed to supervise these types ofsensors.

It has been known to use electrical stimulus to apply a current to suchsensors, to measure the sensor's signal over time, and calculate acapacitance value. This capacitance value can indicate that thesensor(s) has (have) degraded beyond a predetermined threshold, or, itcan be an indication the sensor has been removed from the circuit.However, by itself, it does not indicate the sensitivity of therespective electrochemical sensor.

Another prior art method measures an electrical noise in a sensor outputsignal. A trouble condition or indication can be output if the noiselevel falls below a predetermined fixed threshold. This method is basedin a known characteristic; that as gas concentration increases, thesensor(s) not only output a signal indicative thereof, they also exhibitincreased noise. FIG. 1A is a graph of output noise vs. gasconcentration in parts per million which illustrates thischaracteristic. FIG. 1B illustrates exemplary response of anelectrochemical sensor to a pulse of 100 ppm of CO. FIG. 1C illustratesincreasing noise in response to exposure to the CO. However, this methoddoes not teach maintaining the sensitivity. It only provides anindication of a failed sensor relative to a fixed threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph illustrating variations in sensor noise as a functionof parts per million of a selected gas;

FIG. 1B is a graph illustrating increase of sensor output signal inresponse to the presence of a selected gas;

FIG. 1C illustrates high frequency noise variations as the sensorresponds to increasing concentrations of a selected gas;

FIG. 2 is a graph illustrating noise as a function of mass ofelectrolyte of a sensor;

FIG. 3 is a block diagram of an exemplary detector in accordance withthe invention;

FIG. 4 is a flow diagram of one aspect of the resent invention;

FIG. 5 is a flow diagram of another aspect of the present invention; and

FIG. 6 is a diagram of aspects of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

While embodiments of this invention can take many different forms,specific embodiments thereof are shown in the drawings and will bedescribed herein in detail with the understanding that the presentdisclosure is to be considered as an exemplification of the principlesof the invention and is not intended to limit the invention to thespecific embodiment illustrated.

A disclosed embodiment of the invention overcomes the problems withmonitoring the sensitivity of an electrochemical sensor over time. Thereare at least four active components that can be used to determine thecondition of the sensor. These include the noise level in the sensor'soutput signal, the drift in the signal over time, the internalcapacitance of the sensor, and the internal impedance of the sensor.

The sensor noise level will increase as the signal increases relativelyproportionate to ambient gas concentration. When the sensor is detectingambient gas, the increase in noise can be correlated against the signalincrease from the electrochemical gas sensor. A function that combinesthe noise level in the absence of gas and the noise level with gas canbe used to calculate a sensitivity adjustment factor that is applied tothe gas signal to determine the local levels of ambient gas.

The electrical noise in the sensor can be combined with other electricalsignals from the electrochemical sensor to determine the sensitivitythereof. A prediction can be made as to remaining lifetime of thesensor.

The electrical signals from an electrochemical sensor exhibit noise thatis related to the level of sensed gas, see FIGS. 1A-1C. If the sensorelectrolyte dries out, there is less electrical activity to generatenoise and the noise level will fall. FIG. 2 illustrates an exemplaryrelationship between the mass of electrolyte and the noise level with nogas present. However, before the characteristics in this graph areexhibited, the noise level may actually rise during the final stages ofdrying before decreasing. Algorithms in the processor can use theincrease in noise above a normal expected value to anticipate a pendingfault condition.

Relative to FIG. 3, a gas detector 10 which embodies the presentinvention includes an electrochemical sensor 12 which has an output,line 12 a, that is coupled to a pair of operational amplifiers 14, 16.The amplifier 14 provides a buffered output of the signal from sensor 12and is configured as a relatively low pass filter and current-to-voltageconverter, see FIG. 4, which is associated with the output signal fromthe sensor 12. An output 14 a from operational amplifier 14 can becoupled to a sensor signal input port 18 a of a programmable processor18.

Operational amplifier 16 is configured as a high pass filter withadditional gain and responds only to the high frequency noise in thesignal from the operational amplifier 14, line 14 a. The combination ofthe low pass characteristics of amplifier 14 and the high passcharacteristics of amplifier 16 create a band pass for the noise. Thatsignal is coupled, via line 16 a, to a noise input port 18 b of theprocessor 18. Processor 18 thus has access to a concentration signal,line 14 a, and an associated noise signal, line 16 a.

Processor 18 can in turn be coupled via output port 18 c to interfacecircuits 20 as would be understood by those of skill in the art.Circuitry 20 can include an rf antenna, indicated in phantom, 22 forwireless configurations. Alternately, interface circuits 20 can couplesignals to and from a wired medium 24. Detector 10 can thus communicatewith an external alarm system, for example, as disclosed in Tice et al.U.S. Pat. No. 6,320,501 entitled “Multiple Sensor System for AlarmDetermination with Device-to-Device Communications”, assigned to theassignee hereof and incorporated herein by reference. It will beunderstood that neither of the detailed configurations of the interfacecircuits 20 nor the type of medium, wired or wireless, are limitationsof the present invention.

Processor 18 operates in accordance with prestored control software 26which could be stored, for example, in electrically eraseable read onlymemory EEPROM 26 a. The detector 10 can be contained within and carriedby a housing 30 as would be understood by those of skill in the art.

The processor 18 in combination with control software 26 can carry outsignal processing in response to signal inputs, port 18 a and noiseinputs, port 18 b. Exemplary processing is discussed subsequentlyrelative to FIGS. 4, 5.

In order to make a meaningful measurement of the noise level, it isimportant to remove transients from the signal. The transients can beremoved signal processing carried out by processor 18. One exemplaryfaun of such processing selects the smaller of two time sequentialsignal values and uses the smaller value in place as the signal value.FIG. 4 illustrates this processing. Other methods can be used such asaveraging or selecting the smaller of more than two time sequentialsignals.

The processor 18 can now establish the noise level in the signal of line14 a. Different methods can be used. The preferred method is todetermine an average of the maximums of the signal (AvgMAX) and anaverage of the minimums (AvgMIN) of the signal over an extended periodof time. The noise level (NL) can then be established: NL=AvgMAX−AvgMIN.

The extended period of time would be selected as would be understood bythose of skill in the art such to follow drifting in the sensor signalwithout significantly changing NL measurements. If a gas is sensed, thesignals will increase rapidly. However, this can cause an error in thecalculation of NL. The calculation of the NL is the temporarily disableduntil the signal again stabilizes within expected levels. The NL is thenused subsequently to determine the sensitivity of the sensor.

The sensor will normally drift over time as the conditions change. Thedrift range (DR) is calculated over a long period of time to detect whenthe detector is deviating from normal. Normal expected range of driftingcan be stored in memory, such as EEPROM 26 a and the drift rangecompared to the expected range. Variations from the expected range canbe used-in the determination of the sensitivity.

Another indication of the sensor condition can be established bymeasuring the capacitance of the electrochemical sensor 12. This can beaccomplished by coupling an electrical current to the sensor 12 andmeasuring the response signal over time. A capacitance value (C) canthen be calculated and applied later in determining the sensitivity. Itshould be noted that C by itself is not a direct indicator ofsensitivity of the electrochemical sensor. It is an indirect indicatorthat is factored into the functions for sensitivity calculation andsensor life-time.

The determinations of NL, DR, and C can be affected by environmentalconditions such as temperature. Therefore, these values can becompensated according to predetermined relationships as would beunderstood by those of skill in the art. The measurement of humidity andtime can also be used to predict drying of the electrolyte and thusfactored into the function.

The sensitivity of the electrochemical sensor 12 can be determined as afunction of NL, DR, C, and TIME. The addition of the manufacturerprovided sensitivity (FS) information can be used to calculate ansensitivity adjustment factor (SA) such that SA=f{NL, DR, C, TIME, FS}.As the detector degrades in performance, the SA value will increase as anon-linear function.

The SA can be applied to compensate the sensor signals (CSS) backtowards the original factory calibration. One of the TIME relationshipscan include a normal expected degrading of the electrochemical sensorover time due to the eroding of the electrode surfaces. This may be inthe range of 5% change per year and would normally be established by amanufacturer of sensor 12. Another TIME relationship can be theaveraging time of the routines such that transient conditions arefurther reduced and these TIME relationships can range from short termfor NL to long term for DR.

As the SA value increases, it is also be an indication that theremaining sensor life time (SLT) is decreasing. If SLT decreases suchthat it is below a dynamic threshold based upon NL, DR, C, and TIME,then a trouble indication can be generated so the sensor can bereplaced. In the meantime, the processor 18 and control program 26 willcontinue to attempt to maintain the original factory calibration.

As noted above, the signal, line 14 a, will increase with the specifiedgas being present. An alarm can be established based upon predetermineroutines as a function of CSS, gas alarm threshold(s), and TIME asillustrated in FIG. 5.

In accordance with the above, FIG. 5 illustrates steps of an exemplarymethod 100. In a step 102 the output from sensor 12 is obtained viaprocessor 18. In a step 104 the transient signals, as discussed above,are removed therefrom.

In a step 106 a running average of maximum noise signals is established.In a step 108, a running average of minimum noise values is established.

The noise level NL is calculated in step 110 as discussed above. In step112 the average CO signal is established.

A maximum average CO signal over a predetermined time interval isestablished in step 114. In step 116 the minimum of the average COsignal value over the time period is established.

A drift value is established in step 118. The capacitance of the sensor12 can be established by any one of a variety of known methods, step120.

The sensitivity adjustment for the signal could be established as afunction of noise level, drift and capacitance in step 122. In step 124the sensitivity adjustment is established. Compensated sensor signalscan be established in step 126.

In step 128 a determination can be made of remaining sensor lifetime asa function of maximum lifetime under ideal conditions and the previouslydetermined sensitivity adjustment, step 124. If the sensor lifetime isless than a predetermined value, step 130, then a trouble indicator canbe communicated from processor 18 via interface circuits 20 to an alarmsystem wherein detector 10 is installed. Finally, in a step 132, theprocessor 18 can output an alarm indication if a function which is basedon compensated sensor signals and time crosses a gas alarm threshold.

This method can also supervise the connection of circuitry to theelectrochemical sensor. If the sensor is removed from the circuitry, theNL will immediately fall to the level of the circuitry noise. The newmeasured NL level can cause a re-calculation of the SA that then isapplied to the SLT prediction that would likely end up below a functionvalue resulting from N, DR, C, and TIME. This will result in a troubleindication so that the sensor can be serviced to restore operation.

One of the dynamic aspects of the present invention is that multiplefactors can be used to determine the sensitivity adjustment over time.These values of NL, DR, and C can combine differently over timeaccording to a predetermined function to adjust the sensitivity and alsodetermine the SLT so sensor maintenance can be performed when required.A significant change in the value of NL, DR, or C can cause an immediatere-calculation of the SA and SLT. A period of time can also trigger thisre-calculation.

Because the equations are dynamic, no internal predetermined SLTthreshold is used. Rather, new SLT thresholds are calculated on demandafter sensor values are received by the processing routines. The resultis a more robust detector that adjusts itself to the present conditionof the sensor.

The generation of alarm and trouble separately enables the system towhich the detector, such as detector 10, is coupled to exhibit a properresponse to the detector's condition. The output indications can betransmitted in communication messages, different wireless patterns, ordifferent audio patterns which are emitted from the detector 10.

FIG. 6 illustrates aspects of a method 200 in accordance herewith.Electrical noise can be measured in an electrochemical gas sensor, as at202. A predetermined threshold can be established, as at 204. The noiselevel can be compared to the threshold, as at 206. An electronicfunctionality test can be performed, as at 208. The functionality testcan include at least one of applying a test current to the sensor andmonitoring the current out of the sensor after the test current has beenremoved, or, applying a test current to the electrochemical sensor andmonitoring the output of the sensor during application of the testcurrent, or measuring the impedance of the electrochemical sensor andcomparing that impedance to predetermined limits as at 210, andindicating a trouble condition if the electrochemical sensor fails theelectronic functionality test, as at 212.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the invention. It is to be understood that no limitation withrespect to the specific apparatus illustrated herein is intended orshould be inferred. It is, of course, intended to cover by the appendedclaims all such modifications as fall within the scope of the claims.

What is claimed:
 1. A method of determining the functionality of anelectrochemical sensor in a gas detecting apparatus, comprising:measuring the electrical noise level in an output signal of anelectrochemical gas sensor circuit; establishing a predeterminedthreshold; comparing the electrical noise level to the predeterminedthreshold; performing an electronic functionality test of theelectrochemical sensor if the electrical noise level is lower than thepredetermined threshold; and indicating a trouble condition if theelectrochemical sensor fails the electronic functionality test.
 2. Amethod as in claim 1 where the electronic functionality test includesapplying a test current to the sensor and monitoring the current out ofthe sensor after the test current has been removed.
 3. A method as inclaim 1 where the electronic functionality test includes applying a testcurrent to the electrochemical sensor and monitoring the output of thesensor during the application of the test current.
 4. A method as inclaim 1 where the electronic functionality test includes measuring theimpedance of the electrochemical sensor and comparing that impedance topredetermined limits.
 5. A method as in claim 1 where the troubleindication is different than an alarm indication.
 6. A gas detectingapparatus comprising: an electrochemical sensor; circuitry including aprocessor coupled to the electrochemical sensor, the processor receivingsignals from the circuitry indicative of noise in the electrical signalsfrom the electrochemical sensor and establishing a noise valueassociated with the electrochemical sensor; circuitry for comparing thenoise value with a predetermined limit value; electronic test circuitryfor determining a functionality condition of the electrochemical sensor;and output circuitry coupled to the processor to indicate thefunctionality condition of the electrochemical sensor.
 7. A gasdetecting apparatus as in claim 6 where the output circuitry comprisescommunication circuitry.
 8. A gas detecting apparatus as in claim 6where the output circuitry includes a light emitting device.
 9. A gasdetecting apparatus as in claim 6 where the output circuitry includes asound emitting device.